题意 给你两个4位素数a, b 你每次可以改变a的一位数但要求改变后仍为素数 求a至少改变多少次才能变成b
基础的bfs 注意数的处理就行了 出队一个数 然后入队所有可以由这个素数经过一次改变而来的素数 知道得到b
#include <cstdio>#include <cstring>using namespace std;const int N = 10000;int p[N], v[N], d[N], q[N], a, b;void initPrime(){memset(v, 0 , sizeof(v));for(int i = 2; i * i < N; ++i)if(!v[i]) for(int j = i; i * j < N; ++j) v[i * j] = 1;for(int i = 2; i < N ; ++i) p[i] = !v[i];}int bfs(){int c, t, le = 0, ri = 0;memset(v, 0, sizeof(v));q[ri++] = a, v[a] = 1, d[a] = 0;while(le < ri){c = q[le++];if( c == b) return d[c];for(int i = 1; i < N; i *= 10){for(int j = 0; j < 10; ++j) //把c第i数量级的数改为j{if(i == 1000 && j == 0) continue;t = c / (i * 10) * i * 10 + i * j + c % i;if(p[t] && !v[t])v[t] = 1, d[t] = d[c] + 1, q[ri++] = t;}}}return -1;}int main(){int cas;scanf("%d", &cas);initPrime();while(cas–){scanf("%d%d", &a, &b);if((a = bfs()) != -1) printf("%d\n", a);else puts("Impossible");}return 0;}
Prime Path
Description
The ministers of the cabinet were quite upset by the message from the Chief of Security stating that they would all have to change the four-digit room numbers on their offices.— It is a matter of security to change such things every now and then, to keep the enemy in the dark.— But look, I have chosen my number 1033 for good reasons. I am the Prime minister, you know!— I know, so therefore your new number 8179 is also a prime. You will just have to paste four new digits over the four old ones on your office door.— No, it’s not that simple. Suppose that I change the first digit to an 8, then the number will read 8033 which is not a prime!— I see, being the prime minister you cannot stand having a non-prime number on your door even for a few seconds.— Correct! So I must invent a scheme for going from 1033 to 8179 by a path of prime numbers where only one digit is changed from one prime to the next prime.Now, the minister of finance, who had been eavesdropping, intervened.— No unnecessary expenditure, please! I happen to know that the price of a digit is one pound.— Hmm, in that case I need a computer program to minimize the cost. You don’t know some very cheap software gurus, do you?— In fact, I do. You see, there is this programming contest going on… Help the prime minister to find the cheapest prime path between any two given four-digit primes! The first digit must be nonzero, of course. Here is a solution in the case above.1033173337333739377987798179The cost of this solution is 6 pounds. Note that the digit 1 which got pasted over in step 2 can not be reused in the last step – a new 1 must be purchased.
Input
One line with a positive number: the number of test cases (at most 100). Then for each test case, one line with two numbers separated by a blank. Both numbers are four-digit primes (without leading zeros).
Output
One line for each case, either with a number stating the minimal cost or containing the word Impossible.
Sample Input
31033 81791373 80171033 1033
Sample Output
670
，放下一处烦恼，收获一个惊喜；放下一种偏见，收获一种幸福；
